Pretensioners reduce the slack in a belt restraint system by rapidly drawing in a length of belt webbing in the first milliseconds of a crash situation, to secure the vehicle occupant in the correct position in the seat and reduce the residual forward momentum of the occupant traditionally resulting from slack in the belt webbing due to relatively loose winding on the retractor, to bulky clothing and/or to belt webbing stretch.
One example of a known pretensioner uses pyrotechnic means to generate gas to move a piston attached to a cable to tighten the belt by rapidly rewinding the retractor or by pulling back the buckle attachment.
There is a problem with such a pyrotechnic pretensioner in sealing the space between the moving piston and the walls of the cylinder within which it moves, against the large force of the gas generated. Known sealing systems are bulky and difficult to assemble accurately with attendant higher costs of manufacture.
The present invention provides an improved pretensioner.